ON THE CHANGE OF REF1UNGIBILITY OF LIGHT. 401 I take this opportunity of referring to another very interesting paper of M. Becquerel's, entitled " Des effets produits sur les corps par les rayons solaires," which is published in the Annales de Gliimie, torn. rx. (1843), p. 257, and with which I was not acquainted till lately, or I should have referred to it before. This paper contains, among other things, an investigation of the effects of transparent and coloured screens on the luminous, chemical, and phoaphorogenic rays, in which it is shown that, notwithstanding the great difference in the action of a given screen on the three classes of rays when, we study the effect of the incident rays as a whole, its action is the very same when we confine our attention to rays of any one refrangibility. Among the media employed by M. Bccquerel, are some whose absorbing effect I have mentioned in the present paper, as having been determined by methods depending upon the change of refrangibility. In such cases my own results, as might have been anticipated, are in perfect harmony with those of M. Bccquerel. With respect to the results at which I have arrived regarding the nature of the phosphorogenic rays of an electric spark, which are mentioned towards the end of the paper, I have been in a good measure anticipated by M. Becquerel. Yet T do not think that even he was aware that so much of the effect of the spark was due to rays of such high refrangibility. Note B. Art. 105. I have since succeeded, by a particular arrangement, in seeing so far into the "lavender" rays as to make out the groups of fixed lines w, n,p by means of light received directly into the eye, and even to perceive light beyond that *. As to the colour of these rays when they are well isolated, I think the corolla of the lavender gives as good an idea of it as could be expected from the circumstances. They seem to me to want the luminousness of the blue and the ruddiness of the violet. * [The arrangement actually adopted was to form a pure spectrum with a quartz train in the usual way, to isolate and at the same time condense a small portion of the spectrum by a small quartz lens of short focus placed in or near the pure spectrum, and to view the spot of light so formed from some distance behind through a quartz prism applied to the eye. This prism was cut to show practically single refraction, and its office was of course to remove to one side the scattered light belonging to the ordinarily visible spectrum.] m. 26 ^—"'J W.